1. Field of the Invention
The present invention relates to a thin film transistor-liquid crystal display (TFT-LCD) and, more particularly, to a method for driving the TFT-LCD using multi-phase charge sharing, in which source lines of the liquid crystal panel are driven with a low power through charge sharing, to reduce the consumption power of a TFT-LCD driving circuit.
2. Discussion of Related Art
In general, a TFT-LCD is being widely used as a screen for a desk-top computer, TV, computer""s monitor because it has the most excellent properties in a variety of LCDs, such as high image quality similar to that of CRT, high-speed response and soon. A conventional TFT-LCD, as shown in FIG. 1, includes a liquid crystal panel 10 having a plurality of pixels each of which is located at the point where each of a plurality of gate lines GL intersects each of a plurality of source lines SL, a source driver 20 for supplying a video signal to each of the pixels through a corresponding source line SL of the liquid crystal panel 10, and a gate driver 30 for selecting a gate line GL of the liquid crystal panel 10 to turn on plural pixels. Each pixel consists of a thin film transistor 1 whose gate is connected to a corresponding gate line GL and whose drain is connected to a corresponding source line SL, and a storage capacitor Cs and a liquid crystal capacitor Clc which are connected to the source of the thin film transistor 1 in parallel.
The operation of the conventional TFT-LCD constructed as above is described below with reference to the attached drawings. A sampling register (not shown) of the source driver 20 sequentially receives video data items each of which corresponds to one pixel and stores them which correspond to the source lines SL, respectively. The video data items which are stored in the sampling register are transferred to the holding register by the signal of the controller. The gate driver 30 outputs a gate line selection signal GLS, to select a gate line GL among the plural gate lines GL. Accordingly, the plural thin film transistors connected to the selected gate line GL are turned on to allow the video data stored in the holding register of the source driver 20 to be applied to their drains, thereby displaying the video data on the liquid crystal panel 10.
Here, the source driver 20 supplies VCOM, a positive video signal and a negative video signal to the liquid crystal panel 10, to thereby display the video data thereon. That is, in the operation of the convention TFT-LCD, as shown in FIG. 2, the positive video signal and the negative video signal are alternately supplied to the pixels whenever a frame changes in order not to directly apply DC voltage to the liquid crystal. For this, the intermediate voltage between the positive and negative video signals, VCOM, is applied to an electrode formed on an upper plate of the TFT-LCD. When the positive and negative video signals are alternately provided to the liquid crystal on the basis of VCOM, however, light transmission curves of the liquid crystal do not accord with each other, resulting in flicker.
To reduce the generation of flicker, there is employed one of a frame inversion, line inversion, column inversion and dot inversion, shown in FIGS. 3A to 3D, respectively. The frame inversion of FIG. 3A is a mode that the polarity of the video signal is changed only when the frame is changed. The line inversion of FIG. 3B is a mode that the video signal""s polarity is varied whenever the gate line GL changes. The column inversion shown in FIG. 3C converts the polarity of the video signal whenever the source line SL changes, and the dot inversion of FIG. 3D converts it whenever the source line SL, gate line GL and frame change. The image quality is satisfactory in the order of the frame inversion, line inversion, column inversion and dot inversion. A higher image quality requires higher power consumption because the number of the generation of polarity conversions increases in proportional to the image quality. This is explained below with reference to the dot inversion shown in FIG. 4.
FIG. 4 illustrates the waveforms of an odd-numbered source line SL and an even-number source line SL, applied to the liquid crystal panel 10, showing that the video signals of the source lines SL change their polarities on the basis of VCOM whenever the gate line GL changes. Here, when it is assumed that the entire TFT-LCD panel displays gray color, the video signal swing width V of the source lines SL is twice the sum of VCOM and the swing width of the positive video signal or the sum of VCOM and the swing width of the negative video signal. The consumed power at the output terminal of the TFT-LCD when the capacitance of the source line SL is CL is calculated by the following formula.
E=CLxcx9cV2 
That is, the dot inversion consumes a large amount of power because the video signal changes its polarity from (+) to (xe2x88x92) or from (xe2x88x92) to (+) on the basis of VCOM whenever the gate line GL changes.
Furthermore, the conventional TFT-LCD consumes a larger quantity of power to increase the generation of heat in case where its TFT is configured of a polysilicon TFT. Accordingly, the characteristic of the liquid crystal and the property of the TFT are deteriorated due to the heat generated. To solve this problem, there is proposed a method for driving the TFT-LCD in which, in order to supply a desired amount of voltage to the liquid crystal of each pixel, with the voltage of the common electrode being fixed, the source driver supplies both ends of the liquid crystal with a voltage higher than the common electrode voltage in the nth frame, and supplies them with a voltage lower than the common electrode voltage in the (n+1)th frame, the voltages, respectively applied to the pixels placed above the same column line and the pixels placed therebelow, having their polarities different from each other, and the voltages, respectively applied to the pixels placed at the left side of the same row line and the pixels located at the right side thereof, having their polarities different from each other even in the same nth frame.
This TFT-LCD is driven in such a manner that charge sharing is performed with charge sharing time set for every row line for charge sharing, and then a voltage corresponding to video data is applied to each pixel. Since the voltage polarity of odd-numbered pixels of the (Mxe2x88x921)th low line is different from that of even-numbered pixels thereof, odd-numbered source lines are connected to even-numbered source lines through a switching element before a desired amount of voltage corresponding to the video data is applied to the pixels of the Mth row line. By doing so, the source line to which the voltage higher than the common electrode voltage is applied to and the source line to which the voltage lower than the common electrode voltage is applied maintain the maximum voltage at the common electrode through charge sharing. With this charge sharing, the source driving circuit reduces the voltage swing width by half in comparison with that of the conventional circuit, decreasing the power consumed for driving the TFT-LCD. The conventional TFT-LCD using charge sharing, however, connects the odd-numbered source lines SL to the even-numbered source lines SL using a transfer gate for a period of blanking time, to move a part of the charges of the source lines charged with the positive video signal to the source lines charged with the negative video signal to allow them to share the charges. Accordingly, the consumption power is reduced by 50% at most. Furthermore, the conventional TFT-LCD requires a plurality of source drivers in order to realize a higher resolution of VGA class  less than SVGA class less than XGA class less than SXGA class less than UXGA class. This narrows the line pitch, bring about reliability problems.
Accordingly, the present invention is directed to a method for driving the TFT-LCD using multi-phase charge sharing that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for driving the TFT-LCD using multi-phase charge sharing, which solves reliability problem between the source lines thereof due to addition of source drivers for realizing a high resolution, and reduces power consumption.
The present invention provides the method for driving the TFT-LCD using multi-phase charge sharing, whose consumption power is reduced much more than that of the conventional TFT-LCD using charge sharing.
To accomplish the object of the present invention, there is provided a TFT-LCD using multi-phase charge sharing, comprising: a source driver for outputting video data signals each of which corresponds to one pixel through a plurality of source lines; switching elements for multi-phase charge sharing; and an external capacitor, connected between a liquid crystal panel and the source driver, for collecting charges of a source line having a voltage higher than a common electrode voltage and supplying them to a source line having a voltage lower than the common electrode voltage when the source lines are connected thereto.
To accomplish the object of the present invention, there is also provided a method for driving a TFT-LCD using multi-phase charge sharing, in which at least one selection signal is applied to drive the TFT-LCD for a period of multi-phase charge sharing time, the method comprising: a first charge sharing step in which even-numbered capacitors, which have been discharged with a voltage VL during a period of (Nxe2x88x921)th gradation expressing time, are charged with the voltage of an external capacitor, VL+(⅓)Vswing, according to a second selection signal; a second charge sharing step in which odd-numbered capacitors, which have been charged with a voltage VH during the period of the (Nxe2x88x921)th gradation expressing time, are charged with a voltage VL+(⅔)Vswing through charge sharing with the even-numbered capacitors charged with VL+(⅓)Vswing by the first charge sharing, according to a third selection signal; and a third charge sharing step in which the odd-numbered capacitors, which should be discharged with VLduring a period of the Nth gradation expressing time, are charged with the voltage of the external capacitor, VL+(⅓)Vswing, according to a first selection signal.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.